1. Field of the Invention
The present invention relates to a method for processing a thin film. More particularly, the present invention relates to a thin-film processing method which is applied to the fabrication of a thin film transistor for a liquid crystal display, an image sensor, a static random access memory (SRAM), or the like.
2. Description of the Related Art
In recent years, as liquid crystal displays have higher definition and a larger size, an increase in production yield of a thin film transistor (hereinafter referred to as a TFT) used as the driving element is essentially required. This is because a driving section of the liquid crystal display includes a large number of TFTs. The production yield of TFTs largely depends on the uniformity of a semiconductor thin film which constitutes the TFTs, and especially on the uniformity of crystallinity. In order to increase the production yield of TFTs, it is very important to form a thin film with superior uniformity.
As a recent trend, the main semiconductor material of a TFT is being shifted from conventional amorphous silicon (a-Si) to polycrystalline silicon (poly-Si). When a thin film of poly-Si is to be formed, the key technique is the annealing (heat treatment) for changing a-Si to poly-Si. As to the annealing, excimer laser annealing which causes no damage to an inexpensive glass substrate having a low deforming point, and low-temperature annealing (at temperatures equal to or lower than about 600.degree. C.) such as solid phase crystallization are considered as promising methods.
Hereinafter, a thin-film processing method used in a fabrication process of a conventional poly-Si TFT will be described. FIGS. 1A, 1B, 2A, and 2B are plan views of an area in which TFTs are to be formed and the cross-sectional views thereof in the course of the process steps in a prior art example A. FIGS. 3A, 3B, 4A, and 4B are plan views of an area in which TFTs are to be formed and the cross-sectional views thereof in course of the process steps in a prior art example B.
As is shown in FIGS. 1A, 1B, 2A, and 2B, in the prior art example A, the patterning of a semiconductor thin film on a substrate 1 is first performed, so as to form an area 2b in which a TFT is to be formed. Then, the area 2b is irradiated with an excimer laser beam for heating the area 2b. As a result, the area 2b is crystallized (annealed), so as to form an area 3c. Herein, the area 3c is used as a semiconductor layer for a TFT.
In the prior art example B shown in FIGS. 3A, 3B, 4A and 4B, a semiconductor thin film of a substrate 1 (in this case, the semiconductor thin film is formed over the entire surface of a semiconductor wafer excluding the edge peripheral portion of the wafer) is irradiated with an excimer laser beam, so as to form a crystallized semiconductor thin film 3. Next, the patterning of the semiconductor thin film 3 is performed, so as to form an area 3c.
FIGS. 5A, 5B, 6A, and 6B show a thin-film processing method in a prior art example C. In the prior art example C as shown in FIGS. 5A, 5B, 6A, and 6B, on a substrate 1 on which a patterned thin film or a semiconductor device 4 and a semiconductor thin film are formed, the patterning of the semiconductor thin film is performed, so as to form an area 2b in which a TFT is to be formed. Next, the excimer laser irradiation is performed, so as to form a crystallized area 3c.
The prior art example A has problems in that the desired device performance cannot be attained for TFTs and there exist large variations in device performance. This is because an inside portion 3a and an edge peripheral portion 3b of the area 3c in which a TFT is to be formed have a difference in crystallinity. In the excimer laser annealing step, the face of the edge peripheral portion of the semiconductor thin film pattern which is parallel to the substrate is in contact with a vacuum, a gas, or the like. Therefore, a difference in heat dissipation occurs between the inside portion and the edge peripheral portion of the semiconductor thin film pattern, and hence the crystallizing processes are different.
In the experiments conducted by the inventors of this invention, when the crystallization by annealing an a-Si thin film was performed in the excimer laser annealing step, the crystallinity in the peripheral portion having a width of about 2-3 .mu.m measured from the edge of the thin film pattern was inferior to that of the inside portion of the thin film pattern. Accordingly, as a TFT is made in a smaller size, most of the semiconductor thin film pattern has inferior crystallinity. This causes the device performance of the TFT to be deteriorated. Moreover, it is difficult to regulate the portion with poor crystallinity by annealing conditions. This may cause variations in device performance.
The prior art example B can avoid the problem of non-uniformity in crystallinity between the inside portion 3a and the edge peripheral portion 3b of the semiconductor thin film pattern which arises in the prior art example A. However, in the prior art example B, the performance of the TFT may be deteriorated and variations in device performance may occur for other reasons. That is, the annealing is performed over the semiconductor thin film formed on substantially the entire face of the wafer with an excimer laser beam, so that the bow or expansion/shrinkage of the substrate affects the dimensional precision of the TFT.
In the prior art example B, it is difficult to adjust the positions of a device fabricating portion and a portion to which the annealing is actually performed, when the annealing is selectively performed to a portion of the semiconductor thin film. This is because there is no thin film pattern which can serve as a positioning reference.
In the prior art example C, the annealing step required for the fabrication of the TFT often adversely affects the patterned thin film or the semiconductor device 4. This is because when the annealing is performed to the semiconductor thin film with an excimer laser beam, the patterned thin film or the semiconductor device 4 is also irradiated with the excimer laser beam, so that the film may be peeled off and the semiconductor device performance may be deteriorated.